The present invention relates to a method and a device for suppressing essentially periodic interference signals, and in particular to a method and a device for suppressing periodic interference in the audio frequency range, which is caused, for example, by a digital telecommunications system during the transmission of data, and acts, for example, on a mobile telecommunications terminal or an external device such as, for example, a hearing aid.
In a large number of digital telecommunications systems, data is transmitted between a mobile telecommunications terminal such as, for example, a mobile telephone, and an associated base station by way of a pulsed radio frequency signal with a predetermined carrier frequency. For what is referred to as a GSM telecommunications system (Global System for Mobile Communications), the carrier frequency is 900 MHz and a pulse frequency is approximately 217 Hz. In contrast, in the case of a Digital Enhanced Cordless Telecommunications (DECT) system the carrier frequency is 1800 MHz and the associated pulse frequency is 100 Hz. A further standard which is based on GSM is the DCS1800 standard which also operates at 1800 MHz. In digital telecommunications systems, a large number of carrier frequencies with different pulse frequencies are therefore used, for which reason the manufacturers of terminals are increasingly developing what are referred to as dual-band or triple-band terminals for implementing the various standards.
In particular, the pulsed radio frequency signal causes problems in this context. The pulsed radio frequency signal is demodulated, for example, by the nonlinear FET characteristic curve of a microphone which is present in the terminal, and in doing so gives rise to interference in the audio frequency range, some of which is clearly perceptible.
FIG. 1 shows a simplified representation over time of a signal which has been subjected to periodic interference such as is output, for example, at the output of a signal source such as, for example, a microphone, which has been subjected to interference by a pulsed radio frequency signal.
FIG. 2 shows a simplified representation over time of the associated pulsed radio frequency signal or periodic interference signal such as occurs, for example, in GSM or DECT telecommunications systems. According to FIG. 2, in the GSM standard, radio frequency pulses which contain the actual information are transmitted at a time interval T of approximately 4.7 milliseconds. In the DECT standard, this time interval T is 10 milliseconds and corresponds to a frequency of 100 Hz in contrast to 217 Hz in the case of GSM. These periodic interference signals can then act on a printed circuit board, and in particular on a signal source such as, for example, a microphone, resulting in the interference peaks represented in FIG. 1.
Conventional devices and methods for suppressing these periodic interference signals are essentially based on shielding the radio interference by way of, for example, a conductive shielding housing of the signal source or a conductive microphone housing. It is necessary to ensure here that the housing is enclosed as completely as possible. An optimum effect is usually achieved by way of a metallic shield. However, such a shield is costly and takes up a lot of space, in particular in devices such as, for example, a mobile telecommunications terminal and/or a hearing aid.
A further possible way of suppressing these periodic interference signals is to eliminate the line-bound interference by way of filtering.
In this context, interference-suppression capacitors are used which are typically mounted spatially close to the field-effect transistor (FET) of the microphone in order to attenuate the periodic radio frequency interference signal there as much as possible. The selection of the capacitor is particularly critical here since the influence of parasitic inductances increases greatly at high frequencies.
Consequently, an optimum interference suppression is achieved only with a capacitor whose impedance is minimal for the respective frequency of the interference signal. However, a disadvantage here is that such signal sources or microphones which are tuned using capacitors cost significantly more than conventional standard electret microphones. In addition, a new signal source or microphone has to be developed for each new telecommunications terminal or mobile telephone model or else each type of hearing aid, since the hardware environment such as, for example, the printed circuit board layout of the terminal or of the hearing aid, influences the properties of the interference-suppression capacitor. A further disadvantage consists in the fact that a respective interference-suppression capacitor is required for each carrier frequency so that signal sources with two interference-suppression capacitors are necessary for a dual-band device, and signal sources with even three interference-suppression capacitors are necessary for a triple-band device.
The present invention is therefore directed toward a method and a device for suppressing essentially periodic interference signals, permitting simplified and improved interference suppression.